Observation of the oceans and seas

observation of the oceans and seas

The observation of the seas and oceans nowadays relies on the use of a variety of tools. Besides the classical “research vessel”, research is also based on remotely operated platforms, either satellites or marine robots.

Question 1 : Why do we observe the ocean ?

From a boat or a plane, the ocean always looks the same. But it’s not! The ocean is incredibly different according to location and season. There is as much, and likely more diversity in the ocean than on the earth’s continents. The human eye can appreciate this diversity on land, but this is not the case for the ocean.

The reason is that the ocean has a third dimension - depth - that makes it a mysterious world, which remains to be discovered. A diver can descend to 100 m, while the average depth of the ocean is 3800 m. One can easily understand how challenging ocean observations can be!

Observing the ocean to know and understand

One of the reasons to observe the oceans is to better identify their constituents (e.g. living organisms) as well as their main characteristics like temperature or salinity. This knowledge is required for a better understanding of the behavior and the role of the ocean in the regulation of our climate. We already know that the oceans store, transport (thanks to oceanic currents), and exchange with the atmosphere, large quantities of heat, oxygen (O2), and carbon dioxide (CO2). We also know that living organisms play a key role in the fixation of CO2. Nevertheless, our knowledge of these oceanic phenomena and processes remain largely insufficient and we need to develop and intensify dedicated observations. This is essential to better understand and hence possibly anticipate future changes.

Observing, exploring and discovering

One other reason for observing the ocean is driven by the urge to explore and by the secret hope to make a discovery. The ocean indeed occupies the largest part of our planet, yet it remains largely unknown, essentially because it is insufficiently explored. In the same way that a new telescope allows for the discovery of distant planets, new observational tools can explore profound depths or difficult to access (i.e. under ice) oceanic locations and these new explorations can result in new discoveries.

To know more about the oceanic glider ...

The oceanic glider ascends and dives down to 1 km depth in the water column in a sawtooth-like displacement (animation). This movement takes place without the help of any propeller, just by changing the instrument’s volume (like the profiling float). The glider is equipped with sensors similar to the ones found on the rosette and they measure, for example, temperature, salinity or chlorophyll a concentration.

The glider surfaces about every four hours, and transmits its measurements to the laboratory via a satellite phone (the glider has a sim card!). The scientists then access and analyze in real-time the vertical profile of these measurements on computers. Decisions can be taken to modify the glider mission (changing direction, reaching shallower depth, powering additional sensors…). The underwater glider is a relative newcomer for ocean observation. It can carry out successive observations every few meters along the vertical, and every ~1 km over the horizontal, which are short distances, with respect to the ocean’s immensity, that are hardly observable using research vessels. Gliders can realize missions extending several thousands of kilometers. Lithium batteries (~30 kg) provide the energy required for moving, powering the sensors and transmitting the data. A typical glider mission is described here.

The profiling float

To know more about profiling floats...

Every 10 days, the profiling float leaves its parking depth (located between 1000 m and 2000 m) and begins its ascent towards the surface (animation). This displacement that takes about 10 hours, does not require a propeller: like the glider, the profiling float moves in the water column by changing its volume (increase for the ascent and decrease for the descent). During its way towards surface, the float activates its sensors and measures, for example, temperature, salinity, or the concentration in chlorophyll a or oxygen (O2). Once at the surface, the float transmits its data to the laboratory through a satellite phone via a sim card. The scientist can thus monitor in real-time, the vertical profile of the various measurements and decide, if needed, to modify the float mission (e.g. going deeper, profiling more frequently…). The float then dives towards its parking depth where it will drift according to current over a new period of 9-10 days. The international Argo program, which focuses on the physical properties of the ocean (mainly temperature and salinity), operates more than 3000 profiling floats of this type in all oceans of the globe. Argo is now associated to an emerging companion program, Bio-Argo, which is more focused on the observation of ocean biogeochemistry and ecosystems.

To know more about ocean color satellites...

The « ocean color » satellite turns around the earth at an altitude of about 700-800 km. It is the only observation tool that records, every day, the richness of oceanic life at the surface of the global ocean and of regional waters like the Mediterranean Sea. This satellite analyzes the color of the light which is reflected from the ocean and which varies according to chlorophyll concentration and hence of phytoplankton abundance. The life-time of such a satellite is about 10 years which is enough to reconstitute and understand the "history" of phytoplankton from one season to another and from one year to the next.

To know more about oceanographer-mammals...

Scientists had the idea to use animals to measure and observe. This is the case in the Antarctic where elephant seals are sometimes equipped with sensors measuring temperature or fluorescence (that estimates chlorophyll a). These instrumented animals travel hundreds of kilometers and dive regularly, sometimes down to more than 800 m, to look for their food. Every time they come back to the surface to breath, the data they have acquired are transmitted via satellite, just like for a profiling float or a glider. The vertical profile of measurements is thus received in real-time in the laboratory.

The research vessel (RV)

For several decades, the research vessel has been and still remains the basic tool for conducting observations on ocean parameters. Many scientists can be onboard, in particular to take measurements that autonomous robots are not yet able to perform.

To know more about oceanographic RV...

The oceanographic RVs like the "James Cook"or the "Marion Dufresne" can have up to 40 scientists onboard for periods extending up to 2 months. A research vessel can be considered as a “super-floating laboratory”. Thanks to its crane and winches several instruments can be deployed at sea (including gliders and profiling floats). Some instruments or sensors directly take measurements along their descent to depth. Others allow taking water samples with especially designed bottles, particles via pumps, or living organisms with zooplankton nets. Among all these instruments, the rosette is the core instrument for the activities conducted on a research vessel.

To know more about the rosette...

The rosette and associated sensors likely represent the most used instrument on an oceanographic ship. The rosette can have anywhere from 12-36 bottles containing several liters. They are arranged in a circle above a group of sensors. The rosette moves in the water column (sometimes down to 6000 m) thanks to a cable. When descending, the sensors transmit to the ship and through the cable the measurements like the temperature, the salinity, or the concentration in chlorophyll a or oxygen. The vertical profiles of these measurements are displayed in “real-time” on a control screen, which is scrutinized by the scientists. At a target depth, the descent of the rosette is stopped and the ascent can start. During this ascent, the bottles are closed at the specific depths that have been decided by the scientists. Once the rosette is onboard, the “dance” of scientists around it and the sub-sampling of water from the bottles can last up to one hour. They then return back to their laboratory to perform the required analyses on sampled water.

To know more about the remotely-controlled surface drones...

Oceanographers are using more frequently such remotely controlled tools deployed from research vessels to perform meteorological as well as seawater measurements. Thanks to these platforms, the oceanic physical (temperature and salinity), chemical (oxygen) or biological (concentration in chlorophyll a) properties of the ocean surface are measured without being perturbed by the presence of a large ship. These are the advantages of remotely controlled sailing boats and propelled trimarans.

To know more about instrumented moorings...

An instrumented mooring at fixed oceanic locations, like the BOUSSOLE in the Mediterranean Sea or the MOBY near Hawaii, measures physical, optical, biological or chemical properties within the water column. For example, the BOUSSOLE buoy has taken measurements at the same zone of the Mediterranean Sea for more than 10 years. This type of long-term observation corresponds to what scientists call a “time series”. A time series is useful and required to understand how an oceanic zone reacts to environmental factors or stresses like diurnal sun cycles, seasons, or storms and how these evolve in the future as a consequence of climate change. The BOUSSOLE buoy also takes measurements useful for verifying and validating the observations of « ocean color » satellites. These measurements are called “sea truths”. The satellite in orbit around the earth does get older and it is important to verify the measurements over time and possibly to correct them.

Question 3 : How are the different platforms used ?

Platforms are chosen as a function of scientific objectives

Ideally, the oceanographer would like to measure everywhere, all over the ocean surface and at all depths and 24 hours per day! If this could be possible, no interesting phenomena would be missed. The reality is, however, somewhat different and priorities have to be given, to select the observation areas, the duration and the observation repetitions. According to these priorities the different platforms are thus chosen in order to explore specific areas for specific times. The following table gives a summary of the possibilities offered by the various platforms to conduct the most adapted observations.

The platforms used in a combined (synergetic) way

The variety of observation platforms now extends their use in a combined way to address more specific objectives. During an oceanic cruise on a research vessel, a glider can be deployed around the ship, for example, by covering a 5 km x 5 km square to verify that the onboard-realized measurements of a single “point” in the ocean is representative of a larger area. Similarly, just before the ship leaves the area, a profiling float can be deployed which can continue taking measurements for several years and thereby chart the evolution of the area.One can also envisage combining satellite observations over the global ocean but restricted to the top 30 m from the surface with measurements taken from the water column by the floats of the Argo program to progressively create a 3D view of the ocean.